A gas sensor is a sensor for sensing presence of a particular gas, there are, depending on the gas to be sensed by the sensing film of a sensor, a sensor for sensing CO, a sensor for sensing (CH.sub.3).sup.3N gas generated when fish in a refrigerator goes bad, a sensor for sensing CH.sub.3 SH gas generated when vegetables go bad and a sensor for sensing C.sub.2 H.sub.5 OH, etc. In general, what we should take into account as basic requisite a gas sensor has to have is to be compact and low power consumptive, in addition to high sensitivity, excellent selectivity and high speed of response.
Such a gas sensor contains a heater in the element to enhance the sensitivity of a sensing film to sense a particular gas by heating the sensing film to a specific temperature (normally to 200 to 500 deg. C.), while maintaining the lowest possible power consumption.
In order to make the power consumption lower, the material of the heater itself should be highly efficient as a heat generating material, and loss of the heat generated in the heater to outsise should be minimized.
When a heater is to be heated to a specific temperature difference .DELTA.T, in general, the amount of heat loss P to outside is expressed as follows; EQU P=Pm+P.sub.R +P.sub.A,
where, P.sub.m is heat loss through the supporting film of the sensor, PA1 P.sub.R is heat loss due to the radiation, and PA1 P.sub.A is heat loss through circumferential air. PA1 h is thickness of the supporting film, PA1 u is width of the supporting film, and PA1 a is length of the heat generation part.
wherein, since the P.sub.R is relatively very small value, and the P.sub.A is small value caused by the geometry of the heating part, it is possible to reduce the heat loss P just by reducing the P.sub.m.
The heat loss through the supporting film Pm of a sensor can be expressed as follows; ##EQU1## where, K is a constant, .sigma. is heat conductivity of the supporting film,
As can be seen from the equation, to reduce heat loss through the supporting film, either the supporting film should be of low heat conductive material with reduced thickness, or the ratio of the length of the heating part to the width of the supporting film should be adjusted.
A conventional thin film gas sensor fabricated considering the foregoing condition could have reduced the heat loss of a heater by providing, after forming a supporting film, a heater, and a sensing film on one side of a silicon wafer, a window formed by carrying out an anisotropic etching of the other side of the wafer.
A supporting film of a thin film gas sensor exerts a very important influence on the characteristics such as efficiency, reliability, etc. of the sensor, depending on the structure, and the thermal, electrical and mechanical properties of the supporting film.
The supporting film is formed by a silicon wafer having a supporting film formed on one side thereof which is etched from the back thereof in an etching solution until an appropriate thickness thereof is left when the etching is stopped.
Such an etch stop is caused by an exposure of, in most cases, boron doped P.sup.+ type silicon layer, a silicon oxide (SiO.sub.2) film, or a silicon nitride (Si.sub.3 N.sub.4) film.
Therefore, in order to form a supporting film of predetermined thickness, though it is necessary to carry out an anisotropic etching of a silicon wafer having the film formed thereon to an exact thickness, it is difficult to control forming the exact thickness of the supporting film due the occurance of small amount of etching of the boron doped silicon layer (hereinafter called "P.sup.+ -Si") or the silicon oxide film in an anisotropic etching solution (KOH water solution).
However, since the silicon nitride (Si.sub.3 N.sub.4) film is not susceptible to an etching solution at all, if underlayer of the supporting film is formed of the silicon nitride (Si.sub.3 N.sub.4) film, an exact thickness of the supporting film can be obtained.
FIG. 1 is a section of a conventional thin film gas sensor.
Referring to FIG. 1, a thin film gas sensor includes a supporting film 2 having a silicon oxide film 2a, a silicon nitride film 2b and a silicon oxide film 2c deposited stacked on a silicon substrate 1 to a thickness of 2.5 .mu.m, 0.2 .mu.m, 2.5 .mu.m, respectively, and having a NiFe metal alloy deposited on the supporting film 2, which is subjected to a patterning to form heaters 3 and temperature sensors 4.
In this instant, size of the active area a of the heaters 3 is made to be 450 .mu.m.times.450 .mu.m.
After forming the heaters 3 and the temperature sensors 4 on the supporting film 2 as described above, a passivation layer 5 is formed thereon using SiON.sub.x.
After forming gas sensing elements 8 each having a sensing electrode 6 and a sensing film 7 on the passivation layer 5, the back of the silicon substrate 1 is subjected to an anisotropic etching in KOH water solution.
This completes fabrication of a conventional thin film gas sensor 10 having a supporting film 2 formed of deposited, stacked structure of SiO.sub.2, Si3N.sub.4 and SiO.sub.2 thereon.
The characteristics of the heater of a conventional thin film gas sensor is shown in FIG. 2.
It can be shown that a conventional thin film gas sensor consumes 70 mw to heat the heat generation part of the heater thereof to 300 deg. C., about 340 mw/mm.sup.2 of the heat generation part, and resistence of the temperature sensor is about 700 .OMEGA. at 300 deg. C.
The thin film gas sensor fabricated according to the foregoing process using, for the supporting film, single layered film of P.sup.+ -Si, SiO.sub.2 or Si.sub.3 N.sub.4, or multiple layered film of SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 has problems of having a difficulty in forming a supporting film having an exact predetermined thickness due to small amount of etch of P.sup.+ -Si or SiO.sub.2, SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 in KOH water solution during the anisotropic etching, and having a limit in reducing the power consumption of the heater due to P.sup.+ -Si and Si.sub.3 N.sub.4 having relatively high heat conduction, which leads to a greater heat loss.